The invention relates to meteorological instrumentation, and particularly to an improved method and apparatus for real-time detection and quantification of precipitation reaching the earth""s surface at a given point.
Rain gauges and snow gauges are common names for devices designed to quantify precipitation and the winter equivalent of precipitation that reaches the earth""s surface. Various types of rain and snow gauges have been developed to detect and quantify precipitation and its winter equivalent. One example of a precipitation gauge uses a container to collect free falling precipitation for later measurement. In the case of winter precipitation or snow, the snow is collected in a container housing chemicals to melt the snow into a liquid form. In another example of a precipitation gauge, the rain or snow is collected in a container and upon accumulation of a measurable amount, the gauge detects or xe2x80x9ctipsxe2x80x9d under the weight of the melted snow pouring the liquid into a collection container. The weight of the collected sample is converted into a corresponding depth measurement to estimate the total accumulation of precipitation and the precipitation rate over time. In both examples, the precipitation ideally free-falls into the accumulation container at the same rate and in the same quantity as the precipitation would fall in the immediate vicinity of the gauge.
One problem with these gauges, however, is the overall accuracy of the gauge is limited to mechanical resolutions of accumulation. Therefore, a light snowfall or rainfall event can go completely undetected due to evaporation from the gauge before detection or a measurable amount of accumulation occurs. Another related problem with these gauges is the inability to report real-time accumulation. Even during heavy precipitation events, there is a time delay ranging from a few minutes to thirty minutes or more before a measurable sample amount is collected.
To correct these problems, more recent gauges such as the gauge described in U.S. Pat. No. 5,744,711 have been developed to provide real-time detection and measurement of precipitation events. These gauges use a pair of thermal plates housed in a cylindrical tube. A first thermal plate or sensor plate is horizontally positioned in the tube to collect precipitation. A second thermal plate or reference plate is vertically positioned under the first thermal plate to protect it from contact with the precipitation while still allowing exposure to the same atmospheric temperature conditions. The pair of thermal plates are individually heated and maintained at a substantially constant temperature during a precipitation event. The difference in current used to maintain the individual thermal plates at the substantially constant temperature is quantified and converted into the precipitation rate. A fan positioned in the tube under the thermal plates draws air through the tube to prevent a convecting heat plume from developing at the top of the tube.
A first problem with this gauge is inaccuracies in data collection caused by solar radiation. During periods when precipitation is not falling, solar radiation contacting the top thermal plate heats the plate causing the power required to maintain the substantially constant temperature to fluctuate. These power fluctuations cause noise and other inaccuracies in measuring precipitation events.
A second problem with this gauge is capturing the precipitation and preventing it from sliding off the top thermal plate before the melting and evaporation can occur that causes the power fluctuation. This is especially critical during blowing precipitation events where the wind carries the precipitation into the system at an angle.
A third problem with the gauge is that it is large and bulky requiring dedicated mechanical components such as a fan, fan motor and tube, which increase cost and require frequent maintenance. Furthermore, during precipitation measuring in remote locations, it is desired to carry as little equipment as possible. This is especially true in locations accessible only by helicopter or all terrain vehicles.
A fourth problem with the gauge is the inability to differentiate between a blowing precipitation event and a natural precipitation event. A blowing precipitation event is where the precipitation, such as snow, has already fallen to the earth""s surface, but due to windy or gusty atmospheric conditions is being blown about to different locations. A natural precipitation event is where the -precipitation is falling to the earth""s surface for the first time. A natural precipitation event may occur in substantially still or windy atmospheric conditions.
For these reasons, it is desirable to have a precipitation measuring system that accounts for solar radiation, differentiates between different precipitation events, is compact, and prevents precipitation from leaving the system before melting and evaporation can occur.
The precipitation measuring system of the present invention overcomes the problems outlined above and advances the art by providing a hot plate precipitation measuring system that accounts for solar radiation, differentiates between blowing and natural precipitation events, and prevents precipitation from leaving the system before melting and evaporation can occur. In the context of this application, precipitation includes year round precipitation during both winter and summer months. Some examples of precipitation include without limitation, snow, rain, mist, drizzle, fog, freezing rain, freezing drizzle, sleet, and hail. The precipitation can be blowing precipitation, natural precipitation, or a combination of a blowing and natural precipitation.
The precipitation measuring system comprises a top thermal plate generally positioned horizontal to maximize exposure to falling precipitation and includes at least one ridge circumscribing the top surface for capturing precipitation. A bottom thermal plate is positioned directly under the top thermal plate to protect the bottom thermal plate from falling precipitation while still exposing it to the same atmospheric temperature and wind conditions as the top thermal plate. At least one solar radiation sensor is connected proximate the precipitation measuring system to measure both direct and scattered solar radiation. During a precipitation event, the top and bottom thermal plates are maintained at a constant temperature and a power consumption curve for each thermal plate is quantified. The power consumption curves are corrected for heating caused by solar radiation and the precipitation rate is measured by the difference in the corrected power consumption curves for the top and bottom thermal plates.
In another embodiment of the precipitation measuring system the at least one solar radiation sensor is replaced by a precipitation on/off sensor that automatically starts the precipitation measuring system at the beginning of a precipitation event and automatically shuts down the system at the end of the event. In yet another embodiment, at least one other pair of thermal plates is used to determine the occurrence of a blowing precipitation event and a natural precipitation event by measuring the difference in the amount of precipitation contacting the pairs of thermal plates.
One or more of the following features can also be incorporated into the present precipitation measuring system: 1) a stand, balloon or other air-borne device to elevate the precipitation measuring system above the earth""s surface; 2) a de-icing apparatus to prevent ice from forming on the stand and other components; and 3) real-time adjustment of the substantially constant temperature of the thermal plates to accommodate varying precipitation rates.
A first advantage of the present invention is that the operating temperature of the present precipitation measuring system is lower than prior art systems because precipitation is captured and trapped by the top thermal plate. This results in cost savings and reduces the hazards of working with a heated device. A second advantage of the present invention is that it is compact and does not include bulky mechanical components that wear out or are subject to frequent maintenance. A third advantage of the present invention is that de-icing the stand and other components permits increased accuracy in precipitation measurement. A fourth advantage of the present invention is that the real-time temperature adjustment of the thermal plates results in power savings, increased accuracy in precipitation measurement, and prevents overloading during heavy precipitation events.